As is well known, in recent years, optical cables such as optical submarine cables are often installed over long distances along with rapid advance of the optical communication network.
In the optical communication network, an optical signal attenuates during transmission through the optical cable to decrease the S/N of the optical signal. Such a decrease in S/N of the optical signal is prevented by installing repeaters every predetermined distance.
More specifically, each repeater installed at a predetermined distance converts an optical signal received from each optical fiber into an electrical signal at the terminal of an optical cable in a given section, amplifies the electrical signal, converts the amplified electrical signal into an optical signal, and transmits the optical signal to each optical fiber of an optical cable in the next section, thereby preventing a decrease in S/N of the optical signal.
Recently, optical fiber amplifiers for directly amplifying an optical signal have been developed.
This optical fiber amplifier amplifies communication signal light such that an optical fiber with a core doped with a rare-earth element such as erbium is excited by light having a shorter wavelength than that of the communication signal light.
This optical fiber amplifier can be inserted in each optical fiber of the optical cable to easily prevent a decrease in S/N of the optical signal.
It is important to evaluate the characteristics of the optical fiber amplifier when a new optical communication network is constructed or in periodic maintenance and inspection.
In evaluating the characteristics of the optical fiber amplifier, the gain G given by the ratio of the light intensity P.sub.IN of an input optical signal to the light intensity P.sub.OUT of an output optical signal must be measured because the optical fiber amplifier is a kind of amplifier.
As is well known, in the optical amplifier, even if no optical signal is input to the input terminal of the optical fiber amplifier, its optical amplification mechanism causes spontaneous emission, and the spontaneous emission is amplified and output to the output terminal of the optical fiber amplifier.
The amplified spontaneous emission (ASE) acts as noise to an amplified optical signal.
In, therefore, evaluating the characteristics of the optical fiber amplifier, the light intensity P.sub.ASE of the spontaneous emission (ASE) must be measured.
Evaluation of the characteristics of the optical fiber amplifier generally employs a noise figure NF given by equation (1) including the measured gain G and light intensity P.sub.ASE as indices representing the noise resistance performance: EQU NF=P.sub.ASE /(h.multidot..nu..multidot.G.multidot..DELTA..nu.) (1)
where h: Planck's constant
.nu.: light frequency of input optical signal PA1 G: gain PA1 .DELTA..nu.: measurement frequency resolving power width (measurement frequency width) of light intensity measurement device. PA1 modulating light output from a light source into a rectangular optical signal having predetermined ON and OFF periods by a first optical modulator; PA1 applying the optical signal modulated by the first optical modulator to an optical fiber amplifier to be evaluated; PA1 passing the optical signal output from the optical fiber amplifier to be evaluated through a second optical modulator only during a given period in the OFF period of the optical signal modulated by the first optical modulator, thereby measuring a light intensity P.sub.ASE of spontaneous emission in the optical fiber amplifier by a light intensity measurement device; PA1 obtaining an optical loss on an optical path extending from the optical fiber amplifier to the light intensity measurement device using output light from the optical fiber amplifier in a no-input state, and correcting, using the obtained optical loss, the light intensity P.sub.ASE of spontaneous emission in the optical fiber amplifier that is measured by the light intensity measurement device; and PA1 obtaining a noise figure NF of an optical signal in the optical fiber amplifier using a corrected light intensity P.sub.ASE ' of spontaneous emission in the optical fiber amplifier in accordance with the following equation: EQU NF=P.sub.ASE '/(h.multidot..nu..multidot.G.multidot..DELTA..nu.) PA1 h: Planck's constant PA1 .nu.: light frequency of input optical signal PA1 G: gain PA1 .DELTA..nu.: measurement frequency resolving power width (measurement frequency width) of the light intensity measurement device. PA1 modulating light output from a light source into a rectangular optical signal having predetermined ON and OFF periods by a first optical modulator; PA1 applying the optical signal modulated by the first optical modulator to an optical fiber amplifier to be evaluated; PA1 passing the optical signal output from the optical fiber amplifier to be evaluated through a second optical modulator only during a given period in the OFF period of the optical signal modulated by the first optical modulator, thereby measuring a light intensity P.sub.ASE of spontaneous emission in the optical fiber amplifier by a light intensity measurement device; PA1 obtaining an optical loss on an optical path extending from the optical fiber amplifier to the light intensity measurement device using output light from an unpolarized light generator, and correcting, using the obtained optical loss, the light intensity P.sub.ASE of spontaneous emission in the optical fiber amplifier that is measured by the light intensity measurement device; and PA1 h: Planck's constant PA1 .nu.: light frequency of input optical signal PA1 G: gain PA1 .DELTA..nu.: measurement frequency resolving power width (measurement frequency width) of the light intensity measurement device. PA1 switching means arranged between a first terminal for receiving an optical output from the light source, an input terminal of the first optical modulator, an output terminal of the first optical modulator, an input terminal of the second optical modulator, an output terminal of the second optical modulator, an output terminal to the optical fiber amplifier, an input terminal from the optical fiber amplifier, and an output terminal to the light intensity measurement device; and PA1 control means capable of measuring the light intensity P.sub.ASE of spontaneous emission in the optical fiber amplifier by a first switching operation of the switching means, measuring the gain G of the optical fiber amplifier by a second switching operation, and measuring the measurement frequency resolving power width (measurement frequency width) .DELTA..nu. of the light intensity measurement device by a third switching operation.
The characteristics of the optical fiber amplifier can be evaluated by the gain G and noise figure NF.
To evaluate the characteristics of the optical fiber amplifier, a laser beam source 1 and an optical fiber amplifier 2 are conventionally connected to an optical spectrum analyzer 4 via an optical switch 3, as shown in FIG. 14.
The optical switch 3 is first switched to the laser beam source 1 side to cause the optical spectrum analyzer 4 to obtain the light intensity PIN as a function of the light wavelength .lambda. of an optical signal input to the optical fiber amplifier 2 (lower curve shown in FIG. 15).
The optical switch 3 is switched to the optical fiber amplifier 2 to cause the optical spectrum analyzer 4 to obtain the light intensity P.sub.OUT as a function of the light wavelength .lambda. of an optical signal output from the optical fiber amplifier 2 (upper curve shown in FIG. 15).
As a result, the gain G of the optical fiber amplifier is given by equation (2) including the input light intensity P.sub.IN and the output light intensity P.sub.OUT : EQU G=P.sub.OUT /P.sub.IN (2)
As shown in FIG. 15, the light intensity P.sub.ASE of spontaneous emission (ASE) is buried in the light intensity P.sub.OUT of the amplified output optical signal. For this reason, the light intensity P.sub.ASE of spontaneous emission (ASE) is difficult to directly measure.
As a method of measuring the light intensity P.sub.ASE of spontaneous emission (ASE), a level interpolation method, a polarization nulling method, and a pulse method are proposed.
Of the three methods, the pulse method utilizes a relatively long recovery time required to recover to a ground state for light of a metastable rare-earth element such as erbium doped in the core of the optical fiber of the optical fiber amplifier.
That is, in the pulse method, an optical signal input to the optical fiber amplifier is enabled/disabled in a cycle shorter than the recovery time. The light intensity P.sub.OUT of an output optical signal is measured in the ON period, and the light intensity P.sub.ASE of spontaneous emission (ASE) is measured in the OFF period.
FIG. 16 is a block diagram of an optical amplifier evaluation apparatus adopting this pulse method.
Light with a wavelength .lambda. emitted from the laser beam source 1 is incident on a first optical switch 7 via an input terminal 6 of an optical modulation unit 5.
The first optical switch 7 switches the incident light to a second optical switch 8 or a first optical modulator 9 on the basis of an instruction from a controller 14.
As shown in FIGS. 17A to 17E, the first optical modulator 9 modulates the incident light into a rectangular optical signal which is enabled/disabled in a predetermined cycle T.sub.0 of, e.g., 5 .mu.s shorter than the above-mentioned recovery time, and outputs the optical signal to the input terminal of the optical fiber amplifier 2 via an output terminal 10.
The amplified optical signal output from the output terminal of the optical fiber amplifier 2 is input to a second optical modulator 12 via an input terminal 11.
The second optical modulator 12 functions to pass the optical signal only during a partial period T.sub.S of the ON period or a partial period T.sub.A of the OFF period of the optical signal output from the optical fiber amplifier 2.
Either of the periods T.sub.S and T.sub.A is employed in accordance with an instruction from the external controller 14.
The optical signal output from the second optical modulator 12 is input to the second optical switch 8.
The second optical switch 8 selects the optical signal from the first optical switch 7 or the optical signal from the second optical modulator 12 on the basis of an instruction from the controller 14, and inputs the selected one to the optical spectrum analyzer 4.
The optical spectrum analyzer 4 analyzes the spectrum of the input optical signal to obtain the light intensity P for the wavelength .lambda. or light frequency .nu..
In the optical amplifier evaluation apparatus having this arrangement, the optical switches 7 and 8 are first switched to the partner sides.
Light incident on the optical modulation unit 5 from the laser beam source 1 passes through the optical switches 7 and 8 to directly enter the optical spectrum analyzer 4.
The optical spectrum analyzer 4 regards the incident light as light incident on the optical fiber amplifier 2, and measures the light intensity P.sub.IN. The optical switches 7 and 8 are respectively switched to the optical modulators 9 and 12. In the second optical modulator 12, the partial period T.sub.S of the ON period is set.
In this state, light is incident on the optical spectrum analyzer 4 during the partial period T.sub.S of the ON period of the optical signal output from the optical fiber amplifier 2.
The optical spectrum analyzer 4 regards the incident light as light output from the optical fiber amplifier 2, and measures the light intensity P.sub.OUT.
While the optical switches 7 and 8 are respectively switched to the optical modulators 9 and 12, the partial period T.sub.A of the OFF period is set in the second optical modulator 12.
In this state, an optical signal in the partial period T.sub.A of the OFF period of the optical signal output from the optical fiber amplifier 2 is incident on the optical spectrum analyzer 4.
The optical spectrum analyzer 4 regards the incident light as spontaneous emission (ASE) from the optical fiber amplifier 2, and measures the light intensity P.sub.ASE.
The controller 14 obtains the gain G and noise figure NF of the optical fiber amplifier 2 using equations (2) and (1).
In this way, the optical amplifier evaluation apparatus adopting the pulse method measures the gain G and noise figure NF of the optical fiber amplifier 2.
However, the optical amplifier evaluation apparatus shown in FIG. 16 suffers the following problems which should be solved.
To calculate the noise figure NF, the absolute level of the light intensity P.sub.ASE of spontaneous emission (ASE) from the optical fiber amplifier 2 must be measured.
An optical loss on the optical path extending from the input terminal 11 to the second optical modulator 12, the second optical switch 8, and an output terminal 13 in the optical modulation unit 5 of FIG. 16 must be accurately measured to correct the light intensity P.sub.ASE of spontaneous emission (ASE) measured by the optical spectrum analyzer 4.
In general, a laser beam incident on the optical modulation unit 5 from the laser beam source 1 has a polarization plane.
The first and second optical modulators 9 and 12 also have such polarization characteristics as to change the light intensity of an output optical signal upon a change in polarization direction of the input optical signal.
The polarization direction of the laser beam incident on the optical modulation unit 5 from the laser beam source 1 is not always constant but varies.
More specifically, the polarization direction of a laser beam when an optical loss on the optical path is measured, and the polarization direction of a laser beam when the light intensity P.sub.ASE of spontaneous emission (ASE) is actually measured are difficult to always coincide with each other.
Accordingly, in the optical amplifier evaluation apparatus shown in FIG. 16, the light intensity P.sub.ASE of spontaneous emission (ASE) cannot be accurately corrected.
As described above, the gain G of the optical fiber amplifier 2 is calculated from the ratio of the light intensity P.sub.OUT of an output optical signal to the light intensity P.sub.IN of an input optical signal in the optical fiber amplifier 2.
The optical paths of the input and output optical signals between the laser beam source 1 and the optical spectrum analyzer 4 are different from each other.
Accordingly, optical losses on the respective optical paths must be independently obtained.
However, since the optical modulator present on each optical path has the above polarization characteristics, the optical loss on the optical path cannot be accurately obtained.
In the optical amplifier evaluation apparatus shown in FIG. 16, therefore, the measurement precision of the gain G of the optical fiber amplifier 2 decreases.
In spectrum measurement for the light intensity P.sub.ASE of spontaneous emission (ASE), since the light intensity per unit wavelength must be measured, the likelihood ratio of the frequency (wavelength) resolving power width .DELTA..nu. of the optical spectrum analyzer must be set high.
The set frequency (wavelength) resolving power width representing the measurement frequency width of the optical spectrum analyzer 4 must be narrow in order to prevent interference between channels for a wavelength-multiplexed optical signal.
Generally, the narrower the set frequency (wavelength) resolving power width .DELTA..nu. is, the lower the likelihood ratio of the resolving power is.
As a result, in the optical amplifier evaluation apparatus shown in FIG. 16, an error caused by the likelihood ratio of the resolving power is large.